This invention relates to process for producing oligoesters, which are useful as an intermediate for preparing unsaturated polyesters, from isophthalic acid and propylene oxide.
In the present specification "unsaturated polyester" indicates the polyester which is prepared from dicarboxylic acid components containing unsaturated dicarboxylic acid and polyol components and is not yet mixed with a vinyl monomer as a crosslinking agent. The mixture of the unsaturated polyester and a vinyl monomer is referred to as "unsaturated polyester resin" in the present specification.
It is known that unsaturated polyester resins obtained by using isophthalic acid as a dicarboxylic acid component (sometimes hereinunder referred to as isophthalic acid type unsaturated polyester resin) are superior to unsaturated polyester resins obtained by using orthophthalic acid as a dicarboxylic acid component with respect to such properties as water resistance, chemical resistance and mechanical properties. However, the time required for esterification is longer when isophthalic acid is used as a dicarboxylic acid component than when orthophthalic acid is used.
The process for producing isophthalic acid type unsaturated polyesters are of two types; one stage process and two stage process. In the one stage process, all of the dicarboxylic acid components containing isophthalic acid and unsaturated dicarboxylic acid are reacted with all of the polyol components, such as a glycol or glycols, simultaneously. The two stage process comprises reacting isophthalic acid with at least one polyol component, such as a glycol or glycols, until the acid value of the product amounts to less than a definite value, for example less than 30, to form an oligoester which is a precursor for unsaturated polyester, followed by reacting unsaturated dicarboxylic acid or its anhydride with the resulting oligoester. Though the reaction time of the one stage process is shorter than that of the two stage process, it is known that unsaturated polyester resins produced by the one stage process are inferior to unsaturated polyester resins produced by the two stage process with respect to various properties.
It was known in the art that an addition product of an aromatic dicarboxylic acid and an alkylene oxide, namely an oligoester, can be produced by reacting the two components in the presence of an amine compound catalyst, such as a tertiary amine. However, when the oligoester obtained by the above process is utilized as an intermediate for unsaturated polyesters, the resulting polyesters are apt to be seriously discolored, and are limited to reproducibility of various characteristics with reference to the curing step of unsaturated polyester resins and various properties of the cured resins, such as mechanical properties, chemical resistance, water resistance and so on, and additionally they have poor storage stability. So the resins are not of practical use.
Reaction of an aromatic dicarboxylic acid with an alkylene oxide in the absence of any catalyst has been attempted to overcome the above-mentioned shortcoming. In this case the addition reaction between isophthalic acid and an alkylene oxide is hard to realize because of the high melting point of isophthalic acid and poor solubility of isophthalic acid in the reaction medium and simultaneously polymerization of the alkylene oxide itself occurs to form the oligoester having an ether bond to considerable extent. In addition, it takes much time to lower the acid value of the oligoester to the necessary extent. So, it has been believed to be impossible to economically obtain unsaturated polyester resins with suitable characteristics by reacting an aromatic dicarboxylic acid, especially isophthalic acid, with an alkylene oxide in the absence of any catalyst.
The inventors of the present invention have carried out intensive researches on oligoesters obtained by the addition reaction of isophthalic acid and an alkylene oxide, particularly propylene oxide. As a result, a process for producing oligoesters which comprises (a) the step of reacting isophthalic acid with propylene oxide in a reaction medium in the presence of an amine compound catalyst to produce an oligoester, (b) the step of removing volatile components from the oligoester obtained in step (a) by heating the oligoester at the temperature within the range of 180.degree.-300.degree. C. and (c) the step of treating the oligoester obtained in the step (b) with a silica-alumina compound having absorbing property, has been proposed, as exemplified in U.S. Ser. No. 150,691 filed on May 16, 1980 by Miyamoto et al., now U.S. Pat. No. 4,306,056.